The present invention relates to a process for determining drag characteristics of an aircraft and, more particularly, to a process for determining drag characteristics of an aircraft including moving a model of the aircraft between various attitudes.
Drag is a mechanical force that opposes motion of an object through a fluid, such as an aerodynamic force opposing motion of an aircraft moving through air. Drag D on an object can generally be represented by an equation (1):D=FA cos α+FN sinα  (1)
where:                FA=aft force;        FN=normal force; and        α=a pitch angle or angle of attack of the object,as shown in FIG. 1. The aft force FA and normal force FN are referred to as drag variables because they are the force values used to determine drag. The aft force FA experienced by the object 10 is measured along a fore/aft axis AA extending between a forward end 12 and an aft end 14 of the object. The normal force FN is measured along a normal axis AN extending between a bottom 16 and a top 18 of the object 10 and is perpendicular to the fore/aft axis. The normal and axial forces FN, FA are referenced to and move with the object 10. The angle of attack α is the angle between the fore/aft axis AA and horizontal H. Drag D and lift L are generally fixed coordinates coinciding with horizontal H and vertical V, respectively. Because determining drag D using equation (1) requires knowledge of the angle of attack α, a reliable way of determining that angle is needed.        
Conventionally, orientation of objects, such as aircraft models in wind tunnels, has generally been measured using a mechanical linkage (not shown) including a potentiometer or encoder. However this method of measuring orientation suffers from fundamental errors associated with mechanical hysteresis and/or slop. Mechanical hysteresis is a retardation effect of load on structural displacement. Thus, displacement of a structure under a current load is a function of the current load and previously applied loads. Hysteresis may result from, for example, slipping between adjacent parts in the mechanical linkage under deformation or anelastic behavior in linkage materials. Slop is a discontinuous change in structural displacement under load that results between non-rigidly connected linkage parts. Because of the errors of mechanical linkage systems, systems (not shown) including an on-board measuring device, such as an electrolytic tilt sensor or a servo-accelerometer, have been used. An electrolytic tilt sensor includes a body containing an electrolytic fluid and an amount of electric current conducted through the sensor changes as the sensor body is tilted. A servo-accelerometer, or “servo”, has a primary axis and measures an amount of acceleration along that axis. If the sensor is at rest, an amount of acceleration due to gravity acting along that axis indicates the sensor attitude. Output “s” of a servo can be represented by an equation (2):s=g·sinα  (2)
where:                g=acceleration due to gravity; and        α=the pitch angle of the sensor.For example, when servo output “s” is zero, the primary axis of the servo is horizontal and the pitch angle α is zero. As another example, when the servo provides an output “s” of gravity “g”, the servo primary axis is vertical and the angle α is 90°. As yet another example, when the servo provides a readout corresponding to a measurement of ½ g, the servo primary axis at an angle of 30° from horizontal H.        
Electrolytic and servo sensors have various shortcomings rendering them unable to accurately measure object 32 orientation as the object attitude changes. Although electrolytic tilt sensors can be accurate for measuring tilt values close to 0°, they tend to be less accurate when measuring tilt angles further away from 0°. For example, some electrolytic tilt sensors have an accuracy that is generally inversely proportional to the angle being measured so that sensor accuracy decreases as the actual angle of the object is increased. Further, electrolytic tilt sensors are temperature sensitive and produce various outputs for the same non-zero tilt angle under various temperatures. Another shortcoming of electrolytic tilt sensors is their inaccuracy under dynamic conditions. That is, when an electrolytic tilt sensor is moved rapidly from one attitude to another and/or the sensor is vibrated, such as often occurs when high velocity fluid passes over the object 32, the electrolytic fluid profile in the sensor body is temporarily disrupted so the sensor readout is not an accurate indication of the actual sensor angle. The temporary disruption of accuracy continues until the electrolytic sensor fluid has had time to settle. Thus, electrolytic tilt sensors cannot be relied upon for accurately measuring angles when the object is rapidly moving, and/or when the object vibrates, even when the object is positioned at an angle of attack α near 0°.
Servo-accelerometers are inaccurate under dynamic conditions because they, being designed to measure acceleration, cannot distinguish whether a change sensed in acceleration is due solely to a tilt of its primary axis or also due to some acceleration during object movement. Thus, servos also cannot be relied upon when the object is moving and/or vibrating. A system and process for using the system is needed to accurately determine object orientation when the object is being accelerated.